I/O Error in exe files

A while ago, i got the next problem: When I downloaded an .exe or .rar or .zip from the internet (for example an installation file or whatever), I always got an I/O error on any file I downloaded. I could download the file, but when I ran it, I got the error. So I could not install the files.

I got so annoyed that I even reinstalled my windows. But... the probleem occured again, on a clean windows and all drivers and updates and my needed programs installed. Thought it had to do with a program I installed. Again I got so annoyed, that I reinstalled windows a second time, and did not even update it (so only the drivers from my mobo and graka where installed). And guess again (yes you're right), again I got an I/O error on any file I downloaded.

So I downloaded a tool for checking my HD. Oops, forgot I got an I/O error. Downloaded it on another pc, installed it on mine, and ran all tests. But all tests passed.

So now I'm stuck. I don't know what to do anymore. Can someone help me please. Tell me which test I have to run......